These studies concern the factors that limit the sensitivity of the normal eye. The new techniques and knowledge developed should be useful in analyzing sensory deficit in ocular disease. A. One study concerns our ability to see easy-to-see visual stimuli. Most visual experience involves these stimuli yet, to date, there have been no studies that measure this suprathreshold sensitivity. To do this study we shall introduce a novel marriage of two well-known psychophysical procedures, staircase testing and bootstrap testing. B. Other experimetns are designed to see if naturally occurring moment-to-moment random variations in the oculomotor parameters of the eye can influence its sensitivity. The pupil size, vergence angle, accomodative power and direction of gaze are all known to exhibit minute seemingly random variation. Possibly such variations limit our visual sensitivity and if so, we can quantify how much. Once this is known it may be possible to better understand the poor vision that occurs in ocular disease. C. One study aims to tighten our estimate of the number of photons necessary for seeing at the absolute threshold of vision. Present day estimates leave some doubt that absolute threshold performance is fully understood. We shall carry out these studies with the very best idea yet of which stimulus will be most sensitively seen hoping that the threshold estimate is thereby made more accurate. D. A fourth study aims to bring futher clarity to our view of uncertainty in the eye. We know that uncertainty concerning the stimulus to be seen can profoundly influence sensitivity and that certain parts of the retinal are more sensitive than others. We will try to map the extent of uncertainty across the entirety of the central field of the normal eye and also try to gain further evidence concerning the nature of the uncertainty effect so as to be able to distinguish it from excessive summation. E. A fifth area concerns binocular neural interactions and new tests of their existence. F. Finally, we shall examine spatial uncertainty in amblyopia.